1. Field of the Invention
The present invention relates to a liquid crystal display device and a driving method thereof. More particularly, the present invention relates to a liquid crystal display device and a driving method thereof, wherein a viewing angle may be expanded by altering a common voltage applied to liquid crystal cells of a liquid crystal display device.
2. Description of the Related Art
Generally, liquid crystal display (LCD) devices are smaller and thinner than cathode ray tubes (CRTs), consume less power than CRTs, and display images by controlling light transmittance characteristics of liquid crystal material via data signals generated in response to video signals (e.g., television signals).
Of the various types of LCD devices, Active matrix LCD (AM-LCD) are excel in displaying moving images. Accordingly, AM-LCD devices generally include a LCD panel supporting a plurality of gate lines, a plurality of data lines crossing the gate lines wherein a plurality of liquid crystal cells (i.e., pixels), defined by the crossings of the gate and data lines, are arranged in a matrix pattern. Switching devices such as thin film transistors (TFTs) are provided within the liquid crystal cells. Arranged at crossings of the gate and data lines, the TFTs transmit data signals applied to the data lines to a corresponding pixel electrode in response to a scan signal applied to a corresponding gate line. Accordingly, light transmittance characteristics of each liquid crystal cell may be selectively controlled in accordance with a voltage level of the data signals applied to the data lines.
Recent developments have suggested that viewing angles of LCD devices may be increased by adjusting an alignment direction of liquid crystal material within different regions (i.e., sub-pixels or domains) within a liquid crystal cell.
FIG. 1 illustrates a sectional view of a LCD panel in a related art, two-domain twisted-nematic (TN) mode LCD device.
Referring to FIG. 1, the related art LCD panel includes a TFT array substrate 34, a color filter array substrate 32 opposing the TFT array substrate 34, and a layer of liquid crystal material 22 arranged between the color filter and TFT array substrates 32 and 34, respectively.
The TFT array substrate 34 includes a lower substrate 11; a plurality of data lines 14 formed on an insulating layer 12; a plurality of gate lines (not shown) crossing the plurality of data lines 14 and divide the lower substrate 11 into a plurality of liquid crystal cells; a plurality of TFTs (not shown) each consisting of a semiconductor layer (not shown) and a source/drain electrode (not shown); a protective film 10 covering and protecting the thin film transistors; a plurality of pixel electrodes 8 formed within pixel areas of each of the liquid crystal cells on the protective film 10 and connected to respective ones of the TFTs; and a lower alignment film 6a covering the pixel electrode 8.
The color filter array substrate 32 includes an upper substrate 1; a black matrix 2 formed on the upper substrate 1 for preventing light leakage in regions corresponding to the gate line, the data line 14, and the TFT (not shown); a plurality of color filters 4 arranged between the black matrix 2 and opposing the pixel areas; a common electrode 18 formed over the color filters 4; a protrusion 20, made of an organic material such as an acrylic resin, formed on each common electrode 18 and over each color filter 4; and an upper alignment film 6b formed on the protrusion 20.
Generally, when a voltage is applied to the LCD panel, an electric field is generated within the liquid crystal layer 22 that drives the liquid crystal molecules within a liquid crystal cell. Where the upper alignment film 6b is formed on the protrusion 20, a projected area 16 is formed that distorts the electric field generated within the liquid crystal layer 22. Accordingly, a dielectric energy of the distorted electric field orients directors of liquid crystal molecules in the layer of liquid crystal material 22 in desired directions.
Two-domain TN mode LCD devices such as those illustrated in FIG. 1 must be driven using a pixel voltage signal having a voltage level that is higher compared to the common voltage to effect a change in the orientation of the liquid crystal molecules arranged proximate the projected area 16. Due to the presence of the projected area 16, pixel voltage signals used in driving liquid crystal cells of two-domain TN mode LCD devices such as those illustrated in FIG. 1 must be about 2V higher than pixel voltage signals are used in driving liquid crystal cells of general TN mode LCD devices. However, data drivers capable of generating such high pixel voltage signals can be prohibitively expensive. Further, effecting an adequate orientation change in liquid crystal molecules arranged proximate the projected area 16 is more difficult than effecting an adequate orientation change in liquid crystal molecules in other areas of the liquid crystal cell. Therefore, viewing angle characteristics of LCD devices such as those illustrated in FIG. 1 can be undesirably degraded or narrowed, thereby requiring the addition of a wide viewing angle film.
Problems related to narrow viewing angles often arise when LCD devices such as those illustrated in FIG. 1 are driven according to a dot inversion scheme. Upon driving LCD devices according to the dot inversion scheme, the polarity of a pixel voltage signal applied to any one liquid crystal cell is opposite the polarity of a pixel voltage signal applied to all adjacent liquid crystal cells, wherein the polarities are opposite with respect to a common voltage. Further, the polarities of the pixel voltage signals applied to the liquid crystal cells are inverted every frame.
For example, and referring to FIG. 2A, during odd frames, a polarity of the pixel voltage signals applied to liquid crystal cells in even numbered rows of odd numbered columns is negative while a polarity of the pixel voltage signals applied to liquid crystal cells in odd numbered rows of even numbered columns is positive. During even frames as shown in FIG. 2B, a polarity of the pixel voltage signals applied to liquid crystal cells in odd numbered rows of even numbered columns is negative while a polarity of the pixel voltage signals applied to liquid crystal cells in even numbered rows of odd numbered columns is positive.
FIG. 3 illustrates a waveform diagram of the relationship between a pixel voltage signal (Vd) and a common voltage (Vcom) applied to liquid crystal cells of the related art LCD device illustrated in FIG. 1, driven according to the dot inversion scheme.
Referring to FIG. 3, according to the aforementioned dot inversion driving scheme, pixel voltage signals (Vd) having positive and negative polarities are alternately applied to liquid crystal cells as AC-type voltages during a horizontal period while a common voltage (Vcom) is commonly applied to the liquid crystal cells as a DC-type voltage. When a voltage difference between the common voltage Vcom and the pixel voltage Vd is about 5V, an orientation of liquid crystal molecules arranged proximate the projected area 16 of the upper alignment film 6b is not altered. Therefore, a larger voltage difference must be generated. Related art solutions typically employ a data driver capable of generating pixel voltage signals (Vd) having larger voltage levels in order to increase the voltage difference. However, these data drivers can be prohibitively expensive.